Sympathisers with extremist Islamic groups will risk jail under controversial plans to make merely associating with a suspected terrorist a crime.

The move is aimed at stopping those now floating on the fringes of terrorist cells from being sucked further in. It reflects serious concern - both within government and among the moderate Muslim community - about how to tackle disaffected young men attracted by the teachings of fundamentalists, or by British mosques identified as recruiting grounds for al-Qaeda activists.

But it will be highly controversial among civil liberties campaigners, since it would allow people who have committed no crime to be dragged before the courts in a 'guilt-by-association' culture.

Liberal Democrats also warned last night that it could backfire among British Muslims if it led to young men being arrested for little more than mixing with people at their mosque.

'We are targeting support networks, the things that enable terrorism to be perpetrated by other people,' said a source close to the Home Secretary, David Blunkett. 'It is intended to deter people from hanging around the fringes of undesirables.'

Blunkett's plans are modelled on the French offence of 'associating with a wrongdoer', which was brought in to tackle Algerian terrorism, under which people can be held for up to 92 hours without charge.

The case against them is then compiled by a juge d'instruction - investigative judge - who could be cleared to see intelligence reports, a system Blunkett also favours: it has a high conviction rate.

The French investigative judge Jean-Louis Bruguière, nicknamed 'The Sheriff', not only uncovered an al-Qaeda plot to blow up the United States embassy in Paris shortly before the World Trade Centre attack, but also provided a detailed picture of a European terror network, linked to al-Qaeda, stretching throughout France, the Netherlands, Belgium, Spain and Britain. Paradoxically, many of the North Africans he targeted moved on to the UK as a result.

Under Blunkett's plans, 'associates' of terrorists would initially face a civil court order - like the anti-social behaviour orders slapped on unruly teenagers - banning individuals from contact with named terror suspects. This would be intended as a deterrent: disobedience would become an offence punishable by jail.

'Association' could cover not only meeting in person, but communicating via email or telephone, or even fundraising: sources close to the Home Secretary said, however, there would have to be evidence of some suspicious intent, rather than merely socialising.

There has been alarm in security circles that, despite lessons learnt post-9/11 about tracking connections between the loosely affiliated terror cells making up al-Qaeda, neither British nor Spanish intelligence services managed to prevent the Madrid bombing.

Mark Oaten, the Lib Dem home affairs spokesman, said he appreciated Blunkett's difficult 'juggling act' over terrorism, but added: 'This particular measure could backfire by alienating the Muslim community. It could lead to individuals being questioned if they had been to somebody's house or a religious festival or something where there were suspects. It could cause all sorts of tensions.'

Natalia Garcia, solicitor for two terror suspects currently being held in Woodhill prison, Milton Keynes, said the French system had already produced severe miscarriages of justice: 'As judges link one person to another, the process becomes completely circular. It is guilt by association.'

A report by the International Federation for Human Rights into a spate of anti-terrorist cases involving Algerians in the late Nineties concluded that the French system violated the European Convention on Human Rights, adding that it had 'inflicted grave, often irreparable damage on their victims'.

French juges d'instruction also have a reputation for agonisingly slow, bureaucratic investigations.

The debate comes as the leader of the Roman Catholic Church in England and Wales, Cardinal Cormac Murphy O'Connor, warns in an Easter message today that politicians must tackle the root causes of terrorism, including poverty, rather than just confronting it head on.

'If the Western world were to devote itself in a very real and sacrificial way to helping the two thirds of the world and especially those parts of the world that live in gross poverty, and did it in a way that actually denied themselves, then I think we'd have a more peaceful world,' he will tell GMTV's Sunday programme.

'I think that some of the threat of terrorism comes out of countries that feel that they have not been treated justly. But terrorism itself is a terrible evil.'

The new counter-terrorism measures rushed through after the 11 September attacks must be reviewed in 2006, and Blunkett has already launched a major public debate over what should replace them.